Nitrogen is often the rate-limiting element in plant growth, and all field crops have a fundamental dependence on exogenous nitrogen sources. Nitrogenous fertilizer, which is usually supplied as ammonium nitrate, potassium nitrate, or urea, typically accounts for 40% of the costs associated with crops, such as corn and wheat in intensive agriculture. Increased efficiency of nitrogen use by plants should enable the production of higher yields with existing fertilizer inputs and/or enable existing yields of crops to be obtained with lower fertilizer input, or better yields on soils of poorer quality. Also, higher amounts of proteins in the crops could also be produced more cost-effectively.
Plants have a number of means to cope with nutrient deficiencies, such as poor nitrogen availability. They constantly sense nitrogen availability in the soil and respond accordingly by modulating gene expression. Although more is being discovered about nitrogen and the components involved in regulating its uptake and utilization, much is still unknown about many of these complex interactions. For this reason, it is interesting when a gene of known or unknown function is shown to have a nitrogen response, as it opens up new possibilities and insights into nitrogen utilization and nitrogen use efficiency in a competitive environment (i.e. low and/or high nitrogen).
Plants have a number of means to cope with nutrient deficiencies. One of the most important mechanisms for doing this is to sequester or store nitrogen in times of abundance to be used later. A class of proteins likely to be involved in this process is peptide transporters. There are few published reports about plant peptide transporters indicating that they play an unexplored role in plant growth and development. Peptide transporters are carrier-mediated, energy dependent transporters. Peptides, which have been internalized to the cell, are broken down into amino acids, which are in turn are used as sources of nitrogen and carbon. Over-expression of a peptide transporter may better provide nitrogen to a plant, thus giving it an advantage in competitive nitrogen (N) environments. Use of a nitrogen assimilation inhibitor as a representation of this competitive environment provides a useful screen for candidates which have better nitrogen use efficiency (NUE). This screen provides a clear-cut method to identify N candidates in that it eliminates the subjective nature of limiting N screens based on slight increases in size and greenness.